An extremely spherical (or flat) wavefront is needed for diffraction-limited (i.e., perfect) optical imaging; a wavefront with phase errors less than a quarter of a wavelength is generally desired in such usage. In most instances, however, such imaging is less than perfect due to phase distortions present in the final wavefront. For example, when viewing scenes through the atmosphere, variations in refractive index due to pockets of hot and cold air can distort a wavefront significantly, blurring images and limiting the resolution. Adaptive optics is a technique which can correct for these aberrations3,4. (Numbers such as these refer to the list of references included at the close of this specification. Each reference in this list is hereby incorporated by reference herein.) The adaptive optics process involves detecting the shape of a distorted wavefront and then applying the inverse error to return the wavefront to a perfect surface.
Wavefront sensors are devices capable of determining how the phase of the observed wave differs from a perfectly flat or spherical ideal wavefront. In typical cases of wavefront analysis, the wavefront sensors are designed to extract phase information that is generally then reduced into complex mathematical terms. With such complex formulae, intensive computations are often required to dynamically characterize the wavefront. Such wavefronts may however change very rapidly. In the case of an adaptive optics application such as a telescope, such calculations may need to be performed hundreds of times a second for thousands of points over the wavefront. Such calculations require dedicated circuitry and/or extremely fast data processors.
There are several different techniques which have been developed for wavefront sensing. Some of the more commonly adopted methods include:
An interferometric sensor (IS): interferometry is a means of extracting phase information by mixing one wavefront with another and using the difference signal as a measure of wavefront error1. The result is usually some form of contour map of the surface of the wavefront. This map can be analyzed to give an absolute phase measurement. The general process can be divided into two possibilities:
Phase mixing with a “reference” wave. In this case, an interference pattern is produced by combining the unknown wavefront with a wavefront of predetermined phase (typically a diffraction-limited plane wave). The unknown wavefront can be determined by finding the difference between the two. This technique is primarily useful for coherent wavefront sources or extremely bright incoherent sources.
Self referencing. In this case the unknown wavefront is divided into two beams—one of which is modified (e.g. being shifted laterally) before being recombined. The interference can then be analyzed to determine the global phase of the original beam. A pyramidal sensor11,17 is one example of a wavefront sensor using this technique. U.S. Pat. No. 4,474,467 discloses an example of this type of wavefront sensor; this patent and each of the other patents identified in this document is hereby incorporated by reference herein.
A Shack-Hartmann-type Wavefront Sensor (SHWFS)6,18,22,23,27: this is a method by which a wavefront is divided into small sections (either using holes or lenslets or micromirrors). The local slope of the wavefront over each subaperture used is calculated (e.g. by focal shift). With the assumption of continuity, the final wavefront is generated by piecing together the slopes of each sub-aperture. Several variations of this type of sensor are included in U.S. Pat. Nos. 4,141,651, 4,399,356, 4,725,138, 5,493,391, 5,912,731, 5,936,720, 6,052,180, 6,130,419, 6,184,974, 6,199,986, 6,299,311 and 6,396,588.
A Phase Diversity Sensor (PDS)7,13-16,20,21: this method requires the unknown wavefront to be imaged at two planes in the vicinity of the pupil of the instrument. By analyzing the differences in the two beam profiles, the initial wavefront error can be determined. Many different methods have been discovered to produce and analyze the wavefront using this technique. These include curvature sensors based on a simple focusing of the beam with a conventional lens, as well as more complex sensors involving diffraction gratings.
The latter two Phase Diversity Sensor methods are generally considered when referring to “wavefront sensors” as they can operate equally well on beams of coherent or incoherent light as well as at light levels much lower than those required for interferometry. However, there are resolution limits on wavefront sensing, and it is computationally intensive to determine an entire surface map of the wavefront. Ideally one would like to have a method of sampling wavefronts at high resolution and low light levels with minimal computational requirements to permit operation at high speeds (large bandwidths). The present invention introduces a completely new method for analyzing a wavefront.